A dynamoelectric machine, such as an AC induction motor, typically includes a rotor core which, in one known form, has opposed substantially planar end surfaces and a substantially cylindrical, longitudinally extending body portion. The rotor core also has a rotor shaft bore and a plurality of rotor bar slots. The rotor bar slots sometimes are referred to as secondary conductor slots.
The above described rotor core typically is formed by a plurality of steel laminations. More specifically, each lamination is stamped from a steel sheet, and has a central opening and a plurality of spaced, radially arranged openings adjacent the lamination outer periphery. The laminations are arranged in a stack so that the openings at the outer periphery of the laminations are aligned to form rotor bar slots and the central openings are aligned to form the rotor shaft bore.
To complete the rotor formation process for a standard cast aluminum type rotor, rotor bars are cast in the rotor bar slots and end rings are cast at the opposing ends of the core using, for example, an aluminum casting process. The rotor bars typically extend through the slots and the end rings "short" the bars together at the ends of the rotor core. A rotor shaft extends into the rotor shaft bore and is secured to the rotor core by any suitable process, such as, for example, interference fit or keying. Such a rotor sometimes is referred to in the art as a "squirrel cage" type rotor.
The above described cast aluminum type rotor provides many advantages including low cost and simplicity. Limitations of the cast aluminum type rotor, particularly for larger motors, are associated with operation at high temperatures and/or at high speeds, due for example to differential dimensional growth between the steel rotor core and the aluminum rotor bars and end rings. More specifically, as the rotor temperature and/or speed increases, the steel rotor core expands radially at a rate and to an extent different from the expansion rate and extent of the aluminum rotor bars and end rings. This differential radial expansion results in stresses on the rotor. One high stress region of particular concern is the interface region between the rotor end rings and the rotor bars at the outermost steel core lamination. If the stresses become substantial, it is possible that end rings could break away from the rotor bars and the rotor could fail, or at least operate less efficiently than desired.
Further, to operate a cast aluminum type rotor at a high temperature and/or a high speed, the rotor components typically must be sufficiently large to withstand high thermal and centrifugal stresses. Of course, increasing the size of the rotor for increased strength reasons increases motor cost since more material is required. In addition, since a larger rotor also typically is more massive than a smaller rotor, the affects of centrifugal forces and thermal forces are increased, thus offsetting many of the benefits of the larger rotor.
A known alternative construction to the cast aluminum type rotor is generally referred to as a fabricated bar type rotor. The rotor core in a fabricated bar rotor is substantially the same as the rotor core in a cast aluminum rotor. The rotor bar slots in a fabricated bar rotor core, however, generally have more simple geometric shapes, e.g., rectangular, than the geometric shapes of the rotor bar slots in a cast aluminum rotor. In the fabricated bar rotor, extruded bars (typically aluminum, copper, or bronze) are inserted through the rotor bar slots and the bars extend from the ends of the rotor core. End rings are then brazed or welded to respective ends of the bars and "short" the bars together. Since the bars extend from the ends of the rotor core, the end rings are spaced from the rotor core ends.
It is believed that a rotor with bar extensions can be operated at higher temperatures and/or speeds than a similar size and material rotor without bar extensions since the bar extensions allow a redistribution of stresses resulting in lower peak, or maximum, stresses at higher temperatures and/or speeds. Specifically, it is believed that because of the bar extensions, the bars are able to flex slightly between the core laminations and the end rings, which results in a redistribution of stresses and strains and lower peak or maximum stresses in the bars and end rings.
Although a fabricated bar rotor may provide the advantages as explained above, such a rotor has several limitations. For example, in a fabricated bar rotor core, simple geometric shapes are required in order to ensure that the extruded bar can be inserted within the rotor bar slots. Generally, however, the geometric shape of the rotor bar slots affects the electromagnetic characteristics of a rotor including starting torque, starting amps, efficiency, and power factor. By limiting the rotor bar slots to simple geometric shapes, less flexibility is afforded a designer in attempting to design rotor bar slots to satisfy certain rotor operating characteristics or parameters. In addition, in order to insert the extruded bars into the slots, the extruded bars must have a smaller cross-sectional area than the cross-sectional area of the slots in the core. As a result, the extruded bars may be loosely fitted in the slots and during operation, the bars may vibrate, which could lead to failure of the fabricated rotor. Also, in fabricated bar rotors, the rotor laminations must be clamped or interlocked together, which adds extra process steps in the rotor manufacture process. Further, tradeoffs are associated with the materials commonly selected for the rotor bars in a fabricated bar rotor. For example, high conductivity aluminum can be easily cast but is difficult to weld or braze, and high strength copper alloys often lose considerable strength during brazing/welding operations. Fabricated bar rotors also are expensive to manufacture, in terms of both labor and material, especially when compared to costs associated with cast aluminum type rotors.
It would be desirable to provide a rotor having the cost advantages of a cast aluminum rotor with the increased temperature and speed operation ranges of a fabricated bar type rotor. It also would be desirable to provide such a rotor which allows a designer to select complex geometric shape rotor bar slots so that desired electromagnetic characteristics can be achieved.
An object of the present invention is to provide a rotor having increased temperature and speed operational ranges as compared to a similar size cast aluminum rotor.
Another object of the present invention is to provide such a rotor at a lower cost than a fabricated bar rotor having similar temperature and speed operation ranges.
Yet another object of the present invention is to provide such a rotor which is more easily manufactured than fabricated bar rotors.